1. Field of the Invention
The present invention relates to a light source for use in optical communication, optical recording and optical measurement.
2. Description of the Prior Art
Conventionally, semiconductor laser has been used as a light source for optical communications and optical recording systems. However, since semiconductor laser has its oscillation wavelength varied depending on the temperature and the amount of injection current, it oscillates in multiple modes as shown in FIG. 1(a). To cope with this matter in making a light source, the laser chip is combined with a frequency selective external cavity so as to construct a light source which is controlled to oscillate at a specific wavelength as shown in FIG. 1(b). For the external cavity, an echellette grating, having a saw tooth cross-section, has been used.
FIG. 2 shows the structure of the conventional light source. A light beam 19 coming out of one facet of a semiconductor laser chip 1 is collimated by a lens 2, and it is incident on an echellette grating 12. The incident light 19 is dispersed depending on its wavelengths, and a light beam 16 with a specific wavelength determined from the angle of the grating 12 is fed back to the active layer 13 of the semiconductor laser chip 1. The semiconductor laser chip 1 oscillates at the wavelength of the feedback light 16 and emits an output light beam 20 with a stabilized frequency from the other facet. The oscillation frequency of the semiconductor laser chip 1 can be changed by rotating the echellette grating 12. The above-mentioned stabilized light source is described in detail in a publication by R. WYATT, entitled "SPECTRAL LINEWIDTH OF SEMICONDUCTOR LASERS WITH STRONG, FREQUENCY-SELECTIVE FEEDBACK", ELECTRONICS LETTERS, 18th July 1985, Vol. 21, No. 15, pp. 658-659; a publication by Thomas L. Paoli et al., entitled "Single longitudinal mode operation of cw injection lasers by frequency-selective optical feedback", Applied Physics Letters, Vol. 25, No. 12, 15th Dec. 1974, pp. 744- 746; and Japanese Patent Application of Laid-open No. 57-85281.
However, due to the polarization dependency of the echellette grating 12, the conventional frequency stabilized light source has been restricted in the disposition of the semiconductor laser chip 1 and grating 12. The echellette grating 12 has a higher diffraction efficiency for the polarized light which electric field is perpendicular to the groove direction N, i.e., S-polarization, while it has a lower diffraction efficiency for the polarized light which electric field is parallel to the groove direction N, i.e., P-polarization. The semiconductor laser chip 1 has the output light 19 polarized in the Y direction parallel to its pn junction plane, and therefore the components must have been disposed such that the pn junction plane 14 of the semiconductor laser chip 1 is at right angles with the groove direction N of the echellette grating 12. In this arrangement, feedback light beams 15 and 16 from the grating 12 align along the pn junction plane 14, causing the light beams 15 of unnecessary adjacent mode to be incident on the active layer 13 as shown in FIG. 3. This results in the generation of spurious modes as shown in FIG. 1(b) and thus in a degraded wavelength selectivity, adjustment accuracy and wavelength purity.
In another case where the pn junction plane 14 of the semiconductor laser chip 1 is parallel to the groove direction N of the echellette grating 12, a very weak feedback light 16 is incident on the semiconductor laser chip 1 due to P-polarization incidence, resulting in a degraded frequency stability and tunable range. Particularly, when a light beam with a wavelength distant from the gain center of the semiconductor laser chip 1 is fed back, suppression of spurious modes falls significantly and oscillation becomes unstable. Accordingly, a practical light source cannot be realized in this dispositional scheme.